Radar systems



March 9, 1965 G. R. wl-:LTI 3,173,139

RADAR SYSTEMS Filed July 25, 1962 5 Sheets-Sheet l m24 mma NVENTOR March9, 1965 G. R. wELTl 3,173,139

RADAR SYSTEMS Filed July 23, 1962 5 sheets-sheet 2 j A A F1620 FIG. 2b

9| k-Transmintervol R @mim- V FASE@ AMPLITUDE AMPUTUDE AMPA/runsGEORGE/P. wem BY M WM AGENT March 9, 1965 G. R. WEL-rl 3,173,139

RADAR SYSTEMS v Filed July 23, 1962 5 Sheets-Sheet 3 Af bw) l l A l g vl l l HG. 212 Af D u! E d v'- m S (a) 5 FIG. 2m

() J FIG. 2n

D D I L E TIME A I4 14 |4 I4 D B J' DELAY DELAY DELAY C 2 A E 2A 2 4A EI5 Il) l5 122 l |3 l5 Y /NVE/vrof? GEORGE R. WELT/ MJLT/@M AGENT G. R.WELTl RADAR SYSTEMS March 9, 1965 5 Sheets-Sheet 4 Filed July 25, 1962AGENT G. R. WELTI RADAR SYSTEMS March 9, 1965 5 Sheets-Sheet 5 FiledJuly 2s, 1962 C d e rw 5 5 5 G G. mm plo :ll F F F F In il .T I l IH,dg, L. l M.mm mm l. nR.m ,m -;H .l 4 v FIG. 5f

M/\/\ AM H65@ NODFSLQ TIME /VVENTOR GERGE R. WELT/ BY @LM TQM AGENTUnited States Patent O 3,173,113@ RADAR SYSTEMS George R. Welti, Newton,Mass., assigner to Raytheon Company, Lexington, Mass., a corporation ofDelaware Filed duly 23, 1962, Ser. No. 211,593 1u Claims. (Cl. SaS- 111)This invention relates generally to systems in which received signalsare applied to matched lilters to thereby autocorrelate the receivedsignals and improve the detection capability of the system. Moreparticularly, the invention relates to an improved system whereinundesirable products of such autocorrelation are cancelled.

The detection capability of, for example, a radar system can be improvedby increasing the time-bandwidth product of transmitted signals. Thisproduct is a ligure of merit and considers both the range resolution orrange accuracy of the system which is improved by increasing thespectre-bandwidth of the transmitted waveform and also considers thesignal to noise ratio of received signals which can be increased byincreasing the duration or duty cycle of the radar pulses.

Heretofore, the eiective pulse period or time has been increased withoutreducing average transmitted power or without increasing transmittedpeak power. This has been accomplished by the technique oi pulsecompression and has resulted in an increase in the time-bandwidthproduct. In pulse compression radar systems, a series of, for example,phase coded pulses are generated by applying a trigger signal to aiilter network. are transmitted and their echoes are received. Thereceived pulses are then applied to the same iilter network producing animpulse representative of the target.

One pulse compression system, such as described in United States patentapplication Serial No. 25,775 .filed April 29, 1960, by George R. Weltinot only generates a phase coded series of pulses to thereby increasethe duty cycle or the time factor in the time-bandwidth product, butalso increases the bandwidth factor in the time-bandwidth product. Thisis accomplished by forming eac. transmitted pulse of a series of signalseach containing a number of subpulses which vary in frequency. Moreparticularly, the subpulses sequentially increase in frequency by Xedfrequency increments and then decrease in frequency by the sameincrements forming a time versus frequency plot for each signal whichappears as ascending and descending starcases. rlhus, each transmittedpulse consists of a series of signals each containing the same number ofsubpulses divided into two groups, the first group forms the ascendingsteps, and the second group forms the descending steps. Each of thethese signals is referred to as a staircase The autocorrelation functionof each of the staircases is obtained by applying echo signals to thesame ilter networks which were employed to generate the staircases andconsists of a central spike embedded in a noise background. This noisebackground can be reduced by optimum design of the stircase waveforms.

in the above-mentioned application the transmitted pulse is coded byphasing each of the staircase subpulses at Zero or 1r. Moreparticularly, in each staircase, the group of ascending subpulses arephase Zero or 1r, andthe group of descending subpulses are also phasedzero or 1r. Accordingly, there are four states for each of thestaircases; namely, each may include zero phased ascending and zerophased descending, or fr phased ascending and 1r phased descending, orzero phased ascending and 1r phased descending, or 1r phased ascendingand Zero phased descending groups of subpulses. Accordingly, the pulsecoding is sometimes referred to as quaternary. A second autocorrelationis made of this quaternary coded series of These coded pulses 3,113,139Patented Mar. 9, 1965 ice staircase signals which form the transmittedpulse to produce impulses representative of a target.

One difficulty in the above radar systems which employ autocorrelationtechniques to increase the time factor in the time-bandwidth product tothereby increase detection capability is that the autocorrelation of thephase coded pulse of staircase signals normally produces not only acentral impulse indicative of the target, but also produces additionalimpulses or hash impulses arranged symmetrically about the centralimpulse and which add an undesired ambiguity to the ouput of the system.It is one object of the present invention to provide an improved radarsystem wherein these additional undesired impulses which are sometimesreferred to as hash are cancelled and do not appear in the output of thesystem.

ln accordance with one feature ofthe present invention, sequentialtransmitted pulses are coded diierently by, for example, impressing onephase-position code on odd numbered transmitted pulses and anotherphase-position code on even numbered pulses. The echoes of these codedpulses are then autocorrelated and alternate products of autocorrelationare reversed in phase so that by summing sequential products, theabove-mentioned hash impulses cancel, whereas central impulsesreinforce.

In accordance with one embodiment of the invention, an envelope or pulseof IF is applied to a four terminal filter network generating aphase-position coded sequential repetition of the envelope, the codebeing imposed as a zero or 1r phasing of the 1F. The coded pulse istransmitted, and rellections are received and fed to the same l'llternetwork. Switching circuits are provided at the input and output of thefilter network so that sequential transmitted pulses are encodeddifferently and are decoded dilierently, and a phase inverting circuitis coupled to the network so that the phase of the central impulsesproduced by autocorrelation of sequential transmitted pulses are in thesame phase, but the hash impulses are in opposite phase.

1n accordance with one embodiment of the invention, a radar transmit andreceive system such as described in United States patent applicationSerial No. 25,775 which includes an input iilter and a canonic network,both of which are employed during both the transmit and receiveintervals, is further equipped with a switching network between theinput filter and canonic network and with another switching network, aphase inversion circuit and an integrating circuit coupled between theoutput of the canonic network and the radar display. A canonic networkis a form of network which contains the minimum number of elements tomeet a given requirement as deined in the text Introduction to ModernNetwork Syntheses by M. E. VanValkenburg, John Wylie & Sons, Copyright1960, page 136. A specific form of canonic network and a mathematicaldefinition thereof, which network is suitable as an element of theapparatus of the invention, is contained on page 407 of the RETransactions of the Professional Group on Information Theory, Vol; umelT-6, Number 3, lune 1960, in the article entitled Quaternary Codes forPulsed Radar by George R. Welti. ri`he first-mentioned switching networkis controlled by an alternate gating pulse generated in response to PRFtrigger pulses, and the second mentioned switching network is controlledby gating pulses coincident with the transmit intervals of the system.As a result, the autocorrelated echo signals which are produced at thesame repetition rate as the trigger pulses each include central impulseswhich are in the same phase, and these central impulses are eachbracketed between minor hash impulses, the phase of the hash associatedwith adjacent sequential central impulses being in opposite phase.Accordingly,

the autocorrelated signals` when applied to the integration circuitwhich has a time constant equal to the interval between trigger pulsesresults in the reinforcement of central impulses and cancellation of thehash impulses.

In accordance with another embodiment of the invention, a canonicnetwork such as described in the abovementioned United States patentapplication is employed but without the input lilter described in theapplication. In this embodiment, pulses of IF are applied to the canonicnetwork through switching circuits which are synchronized with the PRFof the system. Different outputs of the canonic network are coupled toan antenna and to a display, and this coupling includes a phasereversing circuit and a second switching network synchronized with thePRF of the system and an integration circuit having a time constantequal to the interval of the PRF. The rst and second mentioned switchingcircuits are constructed and controlled so that the pulses of IF, whichare each produced in response to a transmit trigger pulse, when fed tothe canonic network during the transmit interval each produce a groupingof similar pulses which are coded by the phase of the IF. This phasecoded group is mixed to produce RF and transmitted. Echoes are receivedduring a receive interval, mixed with local oscillator frequency toproduce the IF, and fed to lthe canonic network through thefirst-mentioned switching circuit wherein the echoes are autocorrelated.As a result, each of the autocorrelated groups includes a centralimpulse bracketed within minor impulses of hash. The phase of adjacentcentral impulses is the same, whereas the phase of the correspondinghash associated with subsequent central impulses is opposite. Theseautocorrelated echo signals are then applied to a delay line typeintegrating circuit including a delay line having an interval equal tothe interval between transmit trigger pulses. The. integrating circuitserves to add subsequent central impulses because they are in phase, butcancels the hash impulses which are out of phase. The added centralimpulse signals are then applied to a display circuit to indicate thepresence of a tar et.

(gDther features and objects of the present invention will be moreapparent from the following specific descriptions taken in conjunctionwith the drawings in which:

FIG. l is a block diagram of a transmit-receive radar system similar tothat described in the above-mentioned United States patent applicationand including additional switching and phase reversing circuitry and anintegration circuit at the input to the display whereby hash impulses inthe autocorrelation products from the canonic network are cancelled;

FIGS. 2a-2n illustrate waveforms of transmitted signals, autocorrelationproducts, and gating signals to aid in understanding the operation ofthe system shown in FIG. 1;

FIG. 3 illustrates a block diagram of a typical canonic networkincluding three delay elements and which is constructed and operatessubstantially as described in considerable detail in the above-mentionedUnited States patent application;

FIG. 4 is a block diagram of a transmit-receive radar system wherein thetime-bandwidth product ligure of merit is increased primarily byincreasing the period of transmitted pulses and wherein autocorrelationhash is cancelled; and

FIGS. 5a-5g illustrate waveforms of transmitted signals, autocorrelationproducts and gating signals to aid in understanding operation of thesystem shown in FIG. 4.

Turning iirst to FIG. 1 there is shown a block diagram of a system verysimilar to that shown in FIG. l of the above-mentioned United Statespatent application which transmits a pulse consisting of a series ofascending and descending staircases of subpulses in response to each PRFtrigger pulse. Accordingly, in response to each trigger pulse in theoutput of trigger generator l the system produces a train of staircasesignals each consisting of subpulses ascending and descending infrequency in a repetitive manner. The number of staircase signals isdetermined by the number of units in a canonic network 2, and the numberof subpulses in each ascent and each descent is determined by the numberof units in an input iilter 3. The operation of the circuit is verysimilar to that described in considerable detail in the above-mentionedapplication. Furthermore, the canonic network 2 and input lter 3 arepreferably constructed substantially the same as canonic network 11 andinput filter 10 in FIG. 1 of the above-mentioned application with theexception that the canonic network Z described in embodiments of thepresent invention, for the sake of simplicity, includes fewer units thanthe canonic network described with relation to FIG. 1 in theabove-mentioned application. More particularly, the canonic network inthe mentioned application includes four binary weinhted delay units,whereas in the present application it includes only three. Furthermore,the trigger generator in the present invention is controlled by theoutput of a delay line type integration circuit which effectivelysynchronizes the period between triggerpulses so that the period isequal to the interval of the delay line. This is preferred as will beseen from further explanation hereinbelow.

In operation, triggers from trigger generator ll such as shown in FIG.2a are applied via a switch 4 to one input of the input ilter 3 just asalso shown in FIG. 1 of the above-mentioned application. Trigger pulsesare also applied to a gate pulse generator 5 which produces pulsesdefining thetransmit and receive intervals such as illustrated by :thewaveform in FIG. 2c. These gating pulses control switch l so that duringthe transmit interval the switch applies the trigger pulses fromgenerator 1 to the input T of the input filter 3. Input T corresponds tothe output of diode gate 50 in FIG. 3a of the above-identifiedapplication.

In response to the triggers from trigger generator 1, the two parts of astaircase signal are produced from input lter 3, one at terminal a andone at terminal b of the lilter. The signal at terminal a is bestrepresented by a plot of frequency versus time and, as such, appears asan ascending staircase, while a similar plot of the waveform at terminalb appears as a descending staircase commencing in time at the end of theascending staircase.

The signals at a and b of the input filter 3 are applied to similarswitches 6 and 7 which are controlled in an identical manner by theoutput of alternate gate pulse generator 8 which produces a sign-al suchas shown in FIG. 2b in response to the output of gate pulse generator 5.The switches 6 and 7 respond to Ithis waveform so that the switches arealways in the same one position during alternate transmit intervals andare always in the same opposite position during the alternate receiveintervals. More particularly, if the transmit intervals are successivelynumbered integral numbers and corresponding receive intervals have thesame numbers, then during odd transmit intervals signals appearing atterminal a in the output of filter 3 will be applied to terminal A ofcanonic network 2, and signals appearing in terminal b of the output oflter 3 will appear rat terminal B of the canonic network 2. Furthermore,during odd numbered receive intervals signals appearing in terminal a ofinput filter 3 will appear at terminal B of the canonic network, andsignals appearing at terminal b of the input lter will appear atterminal A of the canonic network. During even numbered transmitintervals, however, the reverse occurs and signals appearing interminals a and b of input lter 3 appear in terminals B and A,respectively, of canonic network 2, whereas during even numbered receiveintervals signals appearing in terminals a and b of input filter 3appear at terminals A and B, respectively, of canonic network 2.

For purposes of clarity, a group of subpulses which form the ascendingpart of a staircase signal will be denoted and a group of subpulseswhich form the descending part will be denoted ,8. Accordingly, duringan odd numbered transmit interval, an rx group appears at terminal A ofthe canonic network 2, and a group `appears at Iterminal B of thecanonic network. These two groups are then combined and phase coded inaccordance with inherent characteristics of the canonic network, and asa result, there appears at the output C of canonic network 2 a series ofa and groups of subpulses which are phase coded in accordance with thecharacteristics of the canonic network. That is to say each subpulsewhich forms, for example, an or group will be phased at either Zero or1r denoted herein as plus (-1-) or minus Accordingly, a plot offrequency versus time of the waveform at terminal C during an oddnumbered transmit interval appears as illustrated in FIG. 2d. Adjacentsubpulses which form each of the a and groups shown in FG. 2d are quiteobviously at different frequencies. However, these frequencies are allgenerally referred to as IF to thereby distinguish from the transmittedor RF.

It will be noted that eight pairs of u and ascending and descendinggroups form eight staircase signals appearing at terminal C, whereasonly a single pair were introduced at terminals A and B of the canonicnetwork. The reason for this can be readily illustrated by reference toa block diagram of the canonic network shown in FIG. 3 illustrating thefour terminals of the canonic network, namely input terminals A and Band output terminals C and D with summing circuits coupled as shown withbinary weighted delays 11, 12 and 13 between the upper and lower summingcircuits 14 and 15, respectively. If, for example, a signal at plus(-1-) phase is introduced at terminal A, it will appear at terminal Cwith substantially zero delay. It will also appear at terminal C sevenadditional times, each delayed by sequential integral numbers of the Adelay units at relative phases shown below.

Zero Delay 1A Delay 2A Delay 3A Delay 4A Delay 5A Delay 6A Delay 7ADelay arrangement of plus (-1-) and minus The phasing codes betweenpairs of terminals are as follows:

In view of the above, it should be clear that the time versus frequencyplot of the signal at terminal C during odd numbered transmit intervalsis as shown in FIG. 2d, and during even numbered transmit intervals isat shown Vin FIG. 2c. In these figures, the a and groups Iare indicated,and the phase coding of each of the groups is indicated by a plus orminus If, for example, yan a group is phase coded -l-, this means that agiven frequency subpulse in that group is in the same phase as the samegiven frequency subpulse in another a group displaced therefrom in timeby an integral number of intervals between trigger pulses. These are theintervals between pulses in FIG. 2a.

The composite pulse at terminal C of the canonic network is fed to amixer 16 via a switch 17, the switch 17 being controlled by the gatepulses illustrated in waveform FIG. 2c so that terminal C is coupled tomixer 1d only during the transmit interval. Mixer 16 serves to mix theIF with local oscillator frequency from the local oscillator 18producing a pulse of RF which is fed to a power amplifier 19. The powergain of amplifier 19 is controlled by the output of a modulator 21 whichis in turn controlled by 6 pulses from trigger generator 1. As a result,the modulator applies operating power to the power amplifier only duringthe duration of the pulse appearing at terminal C of the canonic networkwhich is substantially the transmit interval. The amplified pulse isapplied to a conventional duplexer 22 and from there to antenna 23.

An echo of the transmitted pulse is detected by the antenna 23 andcoupled through the duplexer 22 to RF amplifier 24 in the well-knownmanner of a duplexer. The output of the amplifier 24 is fed to a mixer25 wherein the RF echo is mixed with the output of local oscillator 18producing IF equal to those generated in the input filter during thetransmit interval. The IF from mixer 25 is fed to IF preamplifier 27,and the output of the preamplifier is coupled to terminal R of inputfilter 3` via a switch 28. Terminal R of input filter 3 is the same asthe output of diode gate 201 shown in FIG. 3a of the above-mentionedUnited States patent application, and switch 2S serves the same functionas diode gate 201.

The various filter circuits which make up input filter 3 and which weretriggered by pulses from trigger generator 1 to generate the subpulseswhich form the a and staircase groups are closely matched in response tothe received waveform. Accordingly, the individual cx and groups ofsubpulses when applied to terminal R of the input filter during thereceive interval are each autocorrelated, and the autocorrelationproducts appear in the Output terminals a and b of the filter during thereceive interval. The construction of input filter 3 is such that theautocorrelation products of the a group of subpulses appear in outputterminal b of input filter 3, and the autocorrelation products of groupsof subpulses appear in terminal a of input filter 3, which is somewhatthe reverse of what happens during the transmit interval.

During odd numbered receive intervals, the autocorrelation products ofgroups appear at terminal a of the input filter and canbe represented byspikes as shown in FIG. 2f. In FIG. 2f the autocorrelation product ofeach single group is represented by a spike projecting above or below abase line depending depending on whether the impulse is at plus or minusphase, respectively. Aecompanying these impulses there is, of course,some noise or hash such as shown in FIGS. 8W and 8x of theabove-identified patent application and discussed in considerable lengththerein. This type of hash is not discussed in the present applicationbecause it is not the purpose of the present invention .to cope with it.Correspondingly, at terminal b during the odd numbered receive/intervals, the correlation products` of etV groups of subpulses willalso consist of a series of impulses, the phase of each being asindicated by the spikes shown in FIG. 2g. During even numbered receiveintervals, on the other hand (which follow even numbered transmitintervals, when the switches 6 and '7 were both in a different positionthan during odd numbered transmit intervals), autocorrelation productsof a groups appear at terminal a of the input filter, andautocorrelation products of groups appear at terminal b as shown inFIGS. 2k and 2l, respectively. Thus, it can be seen that the same typeof autocorrelation products appear at terminal a during odd numberedreceive intervals as appear at terminal b during even numbered receiveintervals, and, furthermore, the same sort of autocorrelation productsappear at terminal b during odd numbered receive intervals as appears atterminal a during even numbered receive intervals. y

The phase coding and delay characteristics of the canonic network 2 arematched to the received echo signal with respect to the phasecoding ofthe staircase signals. The canonic network is not matched in a frequencysense; it is matched with respect to the position and phase of impulseswhich are fed to it. Accordingly, the canonic network is matched to theautocorrelation products of the et and groups of subpulses and, if theimpulses at the terminals a and b are fed to the proper terminals A or Bof the canonic network 2, the autocorrelation products of these impulseswill appear at terminal D of the canonic network. In short, the productsof the second of two autocorrelation processes will appear at terminalD. The alternate gating signal shown in FIG. 2b which controls theswitches 6 and 7 is designed to couple the outputs of the input lter 3to the canonic network during both odd and even numbered receiveintervals so that the autocorrelation products of the impulses appear atterminal D. Furthermore, these autocorrelation products at terminal Dare such that by merely inverting phase of such products received duringalternate receive intervals, the products received during subsequentintervals can be added in an integrating circuit, and the centralimpulses will reinforce, whereas undesired hash impulses will cancel.

In order to illustrate the above operation, consider the waveforms shownin FIGS. 2h, 2]', 2m and 2n. These illustrate the above-mentioned secondautocorrelation products of a and ,B type groups at terminal D of thecanonic network during odd and even numbered receive intervals. Duringthe odd intervals, the a group product from the canonic network appearsas represented in FIG. 2/1 which shows the relative position and phaseof these products. They include a central impulse 3l positioned withminor hash impulses disposed symmetrically on either side thereof. Theseproducts result during an odd numbered receive interval when aphase-position coded signal which was originally generated by encodingan a group from terminals A-C is decoded from terminals B-D. Moreparticularly, the position-phase code from A-C, as already mentionedabove, is

During the odd number receive interval, this A-C coded signal isautocorrelated between terminals B-D of the canonic network, and theposition-phase coding from terminals B-D is -i- -1- -i- Theautocorrelation products can be determined as follows:

From the above it is clear that the central impulse 31 has an amplitudeof eight units and is in negative phase; the immediately adjacent hashspikes impulses 32 have an amplitude of one, and are also in negativephase; the next two adjacent hash impulses 33 have an amplitude of threeand are in positive phase; the next adjacent hash impulses haveamplitudes of one and are in positive phase, and etc. Similarly, theposition-phase autocorrelation of the type autocorrelation productswhich appear during odd numbered receive intervals at terminal a or" theinput lter produce impulses at terminal D having amplitude, position andphase relationships as indicated in FIG. 2j. This can be demonstrated byautocorrelating la B-C code which is -I- -1- -1- with an A-D code whichis -i- -i- -las shown below:

During the even numbered receive intervals the position phaserelationship of impulses appearing at terminal a of the input filterwhich are the autocorrelation products of type groups are as illustratedin FIG. 2k. At the same time, the autocorrelation products of ,8 typegroups appear at terminal b as indicated in FIG. 2l. During the evennumbered receive intervals, the signal from alternate gate generator 8shown in FIG. 2b operates the switches 6 and 7 so that terminal a of theinput lter couples to terminal B of the canonic network, and terminal bof the input lter couples to terminal A of the canonic network.Accordingly, the o: type correlation products at terminal a of the inputfilter which were position-phase coded B-C or -i- -i- -l- -i- -I- duringan even numbered transmit interval are autocorrelated from tenninal A toterminal D. The code A-D is -1- -I- -l- -l-, and so the products atterminal D are determined as below:

Similarly, the /3 autocorrelation products at terminal b during the evennumbered receive intervals encoded during an even numbered transmitinterval are positionphase coded A-C or -l- -1- -l- -1- This signal isapplied to terminal B of the canonic network and, therefore,position-phase correlated from B-D so that the double correlationproducts at terminal D are determined as follows:

+1 +1 +3 -1-8-1 +3 +1 +1 Accordingly, at terminal D during the evennumbered receive intervals, the double autocorrelation products of the atype groups is as indicated n FIG. 2m and of type groups as indicated inFIG. 2n.

The next step is to combine the signals appearing at terminal D so thatthe central autocorrelation impulses such as 31 of the same frequencyadd in phase and so that the hash impulses such as 32 and 33 of the samefrequency cancel in phase. This is accomplished by reversing the phaseof the autocorrelation products during either the even numbered orduring the odd numbered receive intervals, then delaying all thecorrelation products by an integral number of intervals between triggerpulses. These are the intervals between pulses shown in FIG. 2a. Forthis purpose a phase inversion circuit 34 is coupled between the outputof terminal D of the canonic network, and a switch 35 which iscontrolled by the alternate gate pulses shown in FIG. 2b. Switch 35 iscontrolled by the alternate gate pulses shown in FIG. 2b and serves tofeed phase-reversed a and ,3 correlation products during alternatereceive intervals to amplifier 36 so that the hash impulses of suchproducts produced during adjacent receive intervals are in oppositephase and will cancel when superimposed in time. The correlationproducts are amplified by amplifier 36 and applied to a delay typeintegrating circuit 37. The circuit includes a delay line 38 and asumming input circuit 39 with a closed loop 41 from the output of thedelay line to the input of the summing circuit. The circuit serves todelay all of the products of autocorrelation by the interval of thedelay line and also establishes the period between trigger pulses fromtrigger generator 1 so that the period between such pulses is equal tothe interval of the delay line.

ln operation, each of the autocorrelation products such as illustratedin FIGS. 2h, 2j, 2m and 2n with the products received during odd or evenreceive intervals appropriately switched in phase as mentioned above,are applied to the summing circuit 39 which in turn applies theseproducts to one end of the delay line 3S. The output of delay line 38 iscoupled back by coupling 41 to the input of the summing circuit 39 andadded to the next received products from the post amplifier. Thefeedback coupling 41, however, couples a predetermined fraction of thesignal at the end of the delay line, and each correlation product fed tothe summing circuit from the post amplifier continues to circulate inthe loop, including the delay line 3S, feedback coupling dl, and summingcircuit 39, but decreases in magnitude with each cycle of circulation.As a result, the output of the delay line includes a impulses, each ofwhich is the sum of series of central or correlation impulses all ofwhich are in the same phase and of decreasing magnitude. Likewise, theimpulses appearing in the output of the delay line are each summationsof a series of central correlation impulses all of the same phase and ofdecreasing magnitude. The autocorrelation hash impulses, such as 32 and33 of both a and autocorrelation products cancel in summing circuit 39and do not appear in the output of the delay line. The output of thedelay line may be utilized by applying it to a detector i2 whichenergizes a display 43 which may be a PPI scope, and as mentioned above,this output also triggers trigger generator l to establish the interval.between transmit trigger pulses from the trigger generator.

Turning next to FIG. 4 there is shown a block diagram of atransmit-receive radar system for transmitting pulses generated in thecanonic network such as shown in FIG. 3 and for receiving echoes of thetransmitted signal, selectively phase reversing some and feeding them tothe same canonic network wherein the received echoes are autocorrelated.The autocorrelated signals are applied to a delay type integratingcircuit for cancelling hash and producing an autocorrelation spike orimpulse representative of a target for energizing a display. Waveformdiagrams which assist to understand the operation of the system in HG. 4are illustrated in FlGS. Saz-5g. As shown in FIG. 4, an 1F signal fromgenerator 5l is gated by pulses from coincident gate pulse generator 52which are initiated by signals from gate pulse generator 53 controlledby triggers from trigger generator 54 which determine the PRF of thesystem. This gating is accomplished by switch 55 producing a short pulseof IF from the switch during the transmit interval illustrated in FIG.5c. FG 5d illustrates the envelope of this pulse which is, forconvenience, denoted as being in positive phase. It should be noted thatthe coincident gate pulses which control switch 55 are coincident withthe transmit interval but are of a shorter duration. This is requiredbecause the waveform shown in FlG. 5c! is expanded in the canonicnetwork, but as expanded should not exceed the transmit interval.

The `waveform shown in FG. 5d which appears at the output of switch 55is applied to terminals A or B of the canonic network 56 via anotherswitch 57. Switch 57 is controlled by the pulses shown in FIG. 5bgenerated in alternate gate puise generator 5l in response to pulsesfrom gate pulse generator 53 shown in FIG. 5c. Generator Si might, forexample, include a differentiating circuit and a single input two-stagedip-lop circuit responsive to negative going spikes from thediiferentiating circuit which represent the back end of the pulses shownin FIG. 5c.

The purpose of switch 57 is to feed the signal at terminal 58 to input Aof the canonic network during odd numbered transmit intervals and toterminal B during odd numbered receive intervals and to terminal Bduring even numbered transmit intervals and terminal A during evennumbered receive intervals. The reason for this is apparent in View ofthe position-phase coding characteristics of the cationic network whichare described above. During even numbered transmit intervals, whenterminal 58 is coupled to terminal B, the pulse will appear at terminalC of the canonic network expanded to the form shown in FIG. 5e and coded-l- ,-l- -l- -lwhich, as shown above, is the code from terminals A-C ofthe canonic network. Since there are, for purposes of example, threebinary weighted delay units in the canonic network, the pulse will giverise to a string `of eight phase coded pulses such as illustrated by thewaveform of FIG. 5e at terminal C. The unit delay A in the canonicnetwork is preferably equal to or greater than the interval of the pulsefrom switch 55, and so the total interval of the pulse at terminal Cduring a transmit interval is preferably at least eight times theinterval of the signal introduced at terminals A or E during thetransmit interval.

Switches 6i and 62 are controlled by gate pulses from generator S3 andcouple terminal Cto a mixer 63 during .the transmit intervals andterminal D to an integrating circuit 64 during receive intervals. Duringthe transmit intervals the coded pulse of IF from terminal C is mixed-in mixer 63 with local oscillator frequency from local oscillator 65-to produce a similarly coded pulse of RF. The pulse of RF is fed topower amplifier 66 which is ,energized during only the transmitintervals by means of gain control modulator 67 responsive to triggerpulses from trigger generator 5d. The output of power amplilier 66 isfed to antenna 68 through a duplexer 69.

During receive intervals, echoes of transmitted pulses picked up by theantenna 68 are coupled through the dupiexer to RF amplifier 71, and fromthere to mixer '72 wherein they are mixed with local oscillatorfrequency from oscillator 65 again producing envelopes of IF which areamplified in IF preamplifier 73 and fed to junction 5S. During receiveintervals switch 55 is closed, and, as mentioned above, during oddreceive intervals, switch 57 couples the signal at terminal 53 to inputterminal B of the canonic network, and during even numbered receiveintervals to input terminal A of the canonic network. The network servesto autoco-rrelate these signals, andthe products of autocorrelationappear at terminal D. During the odd numbered receive intervals, thecorrelation products will be an envelope of lF including peaks ofrelative amplitude phase and position as illustrated in FIG. 5f. Duringeven numbered receive intervals the signal envelope at C is asillustrated in FlG. 5g. During all receive intervals, switch 61 couplesterminal D to IF post amplifier flvia a phase determining circuit 85.The circuit includes a switch 86 and phase reversal 87 Ycontrolled bythe alternate' gate pulses shown in FIG. 5b so that the trains ofwaveforms received during sequential receive intervals include centralimpulses which are in the same phase but include hash impulses which arein opposite phase just as already described above with reference to FIG.l and FIGS. 2h and 2m.

The output of IF post amplier 84 is integrated by integration circuit 64including a delay line 89, feedback coupling 9i and summing circuit 92.The interval of the delay line S9 is preferably equal to the intervalbetween trigger pulses from trigger generator 54 so that correspondingcorrelation products of consecutive received intervals combine in thesumming circuit reinforcing central impulses and cancelling hashimpulses, producing at the output of the summing circuit envelopes 0f IFsuitable for energizing a display. These envelopes are apphed to adetector 97 which energizes a display 98 which may be, for example, aPPI display.

This concludes descriptions of a few embodiments of the presentinvention, each related to a transmit-receive radar'system in whichsignals are encoded at least with respect to position and phase, andtransmitted. Echoes of the transmitted Vsignals are fed tothe sameencoding circuits through suitable switching networks controlled in apredetermined manner sothat the desired products of autocorrelation bearone phase relationship, whereas undesired products bear another phaserelationship and may be cancelled while the desired products areretained. These embodiments are intended to illustrate the features ofthe invention and should not limit the scope of the invention as setforth in the accompanying claims.

What is claimed is:

1. A signal system comprising:

means for generating successive trains of encoded signals of whichdifferent alternate groups of trains are encoded differently;

means for transmitting said successive trains of signals;

means for receiving and autocorrelating said transmitted signals;

and means coupled to an output of said autocorrelating means forcancelling some autocorrelation products of successive trains and forretaining other products.

2. A signal system comprising:

means for generating successive trains of encoded signals of whichdifferent alternate groups of said trains are encoded differently;

means for transmitting said encoded trains of signals;

means for receiving and autocorrelating each of said successive trainsof signals;

and delaying and adding means coupled to said autocorrelating means forcancelling some selected products of said autocorrelation and forretaining other products.

3, A signal system comprising:

means for generating successive trains of encoded signals of whichadjacent trains are encoded differently; means for transmitting saidencoded trains;

means for receiving and autocorrelating each of said successive trains;

and integrating means coupled to said autocorrelating means forcancelling undesired products of said autocorrelation while retainingdesired products.

4. A system for transmitting a signal generated in a lter network andfor receiving echoes of said signals and autocorrelating them in thesame lter network comprising:

means generating an envelope of signals;

a lter network;

switching means coupling said generating means to said lter network;

transmitting means coupled to an output of said network;

means for receiving echo signals of said generated envelope of signals;

means coupling said receiver means to said lter network;

means coupled to the output of said lter means for delaying and addingsuccessive received echo signals; gating means coupling said generatingand receiving means to said filter network;

and phase reversing means coupling said filter network to said delayingand adding means responsive to signals synchronized with the initiationof said envelope whereby some autocorrelation products from said lilternetwork are reinforced in said adding circuit and other products cancel.

5. A system for transmitting a signal generated in a filter network andfor receiving echoes of said signals and autocorrelating them in thesame lter network comprising:

means generating an envelope of signals;

a filter network;

switching means coupling said generating means to said lter network;

transmitting means coupled to an output of said network;

means for receiving said echo signals;

means coupling said receiver means to said ilter net- Work;

integrating means responsive to the output of said lter network;

gating means coupling said generating and receiving means to said lternetwork;

phase reversing means coupling said filter network to said integratingmeans;

and means generating signals synchronized with the initiation of saidenvelope for controlling said gating and phase reversing whereby some ofthe autocorrelation products from said filter network are reinforced insaid integrating circuit and other products cancel.

6. In a system for generating and transmitting signals and receiving andautocorrelating said signals;

means for retaining desired products of said autocorrelation whilecancelling undesired products comprising:

means periodically producing initiating signals;

lilter means responsive to each initiating signal including at leastmeans for inverting and delaying to produce an encoded train of signals;

means for transmitting said encoded trains;

receiving means responsive to said transmitted encoded trains;

means coupling the output of said receiver means to said lter means;

means integrating lan output of said lter means;

and gating circuits associated with said lter means and synchronizedwith said initiating signal whereby desired products of saidautocorrelation of said trains of signals are retained in saidintegrating means, whereas undesired products are cancelled.

7. In a system for generating and transmitting signals and receiving andautocorrelating said signals;

means for retaining desired products of said autocorrelation Whilecancelling undesired products comprising:

means periodically producing initiating signals;

iilter means responsive to each initiating signal including at leastmeans for inverting and delaying to produce an encoded train of signals;

means for transmitting said encoded trains;

receiving means responsive to said transmitted encoded trains;

switching means coupling the output of said receiver means to said ltermeans;

means for integrating an output of said ilter means;

second switching means and phase inverting means coupling said filteroutput to said integrating means;

and means producing signals synchronized with said initiating signal forcontrolling said switching means whereby selected products ofautocorrelation in the output of said lter means are reinforced in saidintegrating means, whereas other products are cancelled.

8. A signal system comprising:

means for generating successive pulses of electrical energy;

means for progressively deriving from each of said pulses symmetricallyvarying frequencies;

means for progressively determining sequences of said frequencies toprovide a coded series of signals, the code of adjacent series beingditerent;

means for applying said successive coded series of signals to said meansfor determining sequences to successively decode each of said series;

and means for delaying and adding said successively decoded serieswhereby desired products of said decoding are retained, whereasundesired products are cancelled.

9. A signal system comprising:

means for generating successive pulses of electrical energy;

means for progressively deriving from each of said pulses symmetricallyvarying frequencies;

means for progressively determining sequences of said frequencies toprovide a coded series of signals, the

code of different groups of alternate successive series being different;

means for applying said successive coded Series of signals to said meansfor determining sequences to successively decode each of said series;

means for reversing phase of one of said groups of alternate decodedseries;

and means delaying and adding said decoded successive series wherebydesired products of said decoding are retained, and undesired productscancelled.

10. A signal system comprising:

means for generating spaced pulses of electrical energy;

means for progressively deriving from each of said pulses asymmetrically varying sequence of frequencies;

means for progressively determining sequences of said frequencies toprovide a coded series of signals in response to each of said spacedpulses, the code of adjacent sequential series being different;

means for applying said successive coded series of signals to said meansfor determining sequences to successively produce signals representingthe autocorrelation function of each of said coded series;

means for reversing phase of selected of said signals representingautocorrelation functions;

and means delaying and adding successive of said signals representingautocorrelation functions to cancel undesired products of saidautocorrelation.

No references cited.

15 CHESTER L. JUSTUS, Primary Examiner.

KATHLEEN CLAFFY, Examiner.

1. A SIGNAL SYSTEM COMPRISING: MEANS FOR GENERATING SUCCESSIVE TRAINS FOENCODED SIGNALS OF WHICH DIFERENT ALTERNATE GROUPS OF TRAIN ARE ENCODEDDIFFERENTLY; MEANS FOR TRANSMITTING SAID SUCCESSIVE TRAINS OF SIGNALS;MEANS FOR RECEIVING AND AUTOCORRELATING SAID TRANSMITTED SIGNALS; ANDMEANS COUPLED TO AN OUTPUT OF SAID AUTOCORRELATING MEANS FOR CANCELLINGSOME AUTOCORRELATION PRODUCTS OF SUCCESSIVE TRAINS AND FOR RETAININGOTHER PRODUCTS.